Add option --enable-copy-only to configure script

Add a new option (--enable-configure-only) to configure script. This
option makes sure that files are only copied when installing sysstat and
that nothing else (like activating a service for distro using systemd)
is done. This may be useful when creating sysstat package.

Signed-off-by: Sebastien GODARD <sysstat@orange.fr.fake>